1. Field of the Invention
The present invention relates to analog amplifiers for pre-amplifying low level charge-based signals, and in particular, to amplifier and processor circuits with analog pre-amplifier circuits and analog-to-digital (ADC) conversion circuits.
2. Description of the Related Art
High dynamic signal range is a key parameter for many types of circuits. This is particularly true in the area of flat panel x-ray imaging systems. As is well known in the art, such systems use a detector cassette containing a scintillation layer that absorbs and converts impinging x-ray photons to visible light photons for detection by photosensitive elements that are also within the detector array. As is further well known, such a detector array contains a two dimensional array of microscopic squares referred to as picture elements, or “pixels”. Each pixel includes an addressable photosensitive element, such as a photodiode and switching transistor combination. From such circuitry individual pixel data signals, generally in the form of charge based signals, are provided for amplification and further processing. (Further discussion of this type of imaging system can be found in U.S. Pat. No. 5,970,115, entitled “Multiple Mode Digital X-Ray Imaging System”, the disclosure of which is incorporated herein by reference.)
Present methods of reading out image pixel information from such flat panel detector arrays involves linear conversion of the electrical charge of each pixel to a voltage. Such voltage is then processed linearly and converted to a digital value using a conventional analog-to-digital converter (ADC). Both processes have inherent limitations, however. The process of linear charge-to-voltage conversion and amplification will have limitations due to noise, linearity issues and limited signal saturation levels. The digital conversion process has a limitation on dynamic range, which presently is typically at 14 bits. (Converters capable of more than 14-bit conversion at speeds required for flat panel imaging arrays have limited, if any, availability as commercial products.)
Improvement of the dynamic range of the linear conversion process has been described in U.S. Pat. No. 6,486,808, entitled “Data Signal Amplifier With Automatically Controllable Dynamic Signal Range” (the disclosure of which is incorporated herein by reference). Such a technique involves automatic switching of the charge-to-voltage gain factor to a lower value when pixels with high signal levels are encountered. Pixel values corresponding to these dynamically reduced gain values are identified (e.g., flagged with an additional bit) such that downstream image processing circuitry can detect such pixel values and correct for the reduced gain factor. This allows signal values large enough to otherwise cause saturation of the charge-to-voltage converter, the signal processing path or the ADC to be “compressed”, converted to appropriate digital data and subsequently expanded, or “decompressed”, thereby extending the dynamic range of signals for conversion.
Compensation or improvement of the digital conversion portion of the processing is disclosed in U.S. Pat. No. 5,760,723, entitled “Delta-Sigma Analog-To-Digital Converter Including Charge Coupled With Devices” (the disclosure of which is incorporated herein by reference).
Extending the dynamic range of the charge-to-voltage conversion, particularly those used in present flat panel imaging arrays, is problematic. Significant limitations exist due to electronic noise and non-ideal and non-linear behavior of electronic components. As for the digital conversion, increasing bit depth of an ADC results in significant additional system cost and power dissipation. Further, ADCs capable of more than 14-bit conversion at the requisite speeds are not readily available as commercial products.
While the dynamic gain switching technique of U.S. Pat. No. 6,486,808 succeeds in extending the dynamic range without increasing physical complexity, cost or power dissipation, correction of the compressed pixel values requires precise information concerning the transfer functions of the signal processing electronics in both standard and reduced gain modes. The binary nature of the gain switching decision along with imperfect restoration of low gain pixel values can produce contour artifacts in image regions where the pixel values cross over the gain switching threshold. Moreover, implementation of this technique requires more complex circuitry which is generally less flexible in terms of being adapted to revised application requirements.
Regarding the digital conversion process, extending the dynamic range at extremely low signal levels is very difficult, and generally requires development and use of significantly more complex circuits which are directed to specific product implementations or system requirements and not readily adaptable to revised application requirements. Further, practical implementation is often precluded due to constraints on power dissipation and available space.